Aqueous solutions and dispersions of polyurethane and polyurethane-polyureas are known. Their preparation has been described, for example, in the following documents: DE-PS No. 880,485, DE-AS Nos. 1,044,404, US-PS 3,036,998, DE-PS 1,178,586, DE-PS 1,134,946, DE-AS 1,237,306, DE-OS 1,595,602, US-PS Nos. 3,756,992, DE-OS 2,019,324, DE-OS 2,035,732, DE-OS Nos. 2,446,440, DE-OS 2,651 506, US-PS 3,479,310 and Angewandte Chemie 82, 53(1970). The known aqueous solutions and dispersions are suitable for various applications, including the formation of bonds on any substrates.
When these dispersions are used for bonding non-absorbent substrates such as rubber, metal or soft PVC, the heat activation process is employed to prevent the inclusion of water. In this process, the dispersion adhesive is applied to the substrate and after complete evaporation of water the layer of adhesive applied is converted into an adhesive state by the action of heat.
One major disadvantage of the dispersions known in the art is that the temperature required for activating the adhesive layer is so high that it frequently damages the substrate. Thus, for example, molded shoe soles of thermoplastic rubber cannot be satisfactorily bonded with presently available polyurethane dispersions since the temperatures required for activating the layer of adhesive cause deformation of the material of the shoe sole.
Attempts to reduce the activation temperature by the addition of solvents, plasticizers or resins resulted in an undesirable loss of heat strength of the bonds.
DE-OS No. 2,804,609 (=EP-B-No. 0,003,521) describes the use of adhesives consisting of polyurethane dispersions obtained from polyurethanes which have been prepared from aliphatic diisocyanates. Although these adhesives form readily activated layers of adhesive, the dispersions do not dry uniformly at room temperature but form scales, i.e. they do not cohere. This causes a severe drop in adhesive strength, especially after prolonged action of moisture. It was therefore an object of the present invention to provide a new adhesive based on an aqueous solution or dispersion of a polyurethane which would not have the disadvantages mentioned above.